Chronic Erosive Synovitis can be induced experimentally in rats by systemic administrations of cell walls from certain bacteria, such as group A streptococci. Genetic and hormonal factors determine susceptibility to arthritis. Susceptible inbred strains develop intense mononuclear cell inflammation in periphral joints, liver and spleen. Resistant inbred strains develop mononuclear cell inflammation in peripheral joints, liver and spleen. Resistant inbred strains develop mononuclear cell inflammation in the spleen but not the joints. The cell walls are poorly degraded and can be demonstrated i macrophages at the sites of inflammation for more than 16 weeks. Thus, susceptibility appears to reflect a host defect in mechanisms which limit translocation of the phlogistic cell walls to synovial tissue. The detailed mechanisms underlying this defect are under continuing investigation.